This invention concerns a method for recovering residual hydrocarbons with a miscible solvent flood from a reservoir which has been previously gas flushed. More particularly, the method employs at least one horizontal well to inject miscible solvent and at least one injection well to inject a drive gas to move the miscible solvent through the reservoir.
Horizontal wells have been investigated and tested for oil recovery for quite some time. At present, the use of horizontal wells is usually limited to formations containing highly viscous crude. In the future, horizontal wells will be used more widely for other types of formations. It seems likely that horizontal wells will soon become a chief method of producing tar sand formations and other highly viscous oils which cannot be efficiently produced by conventional methods because of their high viscosity. Most heavy oil and tar sand formations cannot be economically produced by surface mining techniques because of their formation depth.
Various proposals have been set forth for petroleum recovery with horizontal well schemes. Most have involved steam injection or in situ combustion with horizontal wells serving as both injection wells and producing wells. Steam and combustion processes have been employed to heat viscous formations to lower the viscosity of the petroleum as well as to provide the driving force to push the hydrocarbons toward a well.
A system of using parallel horizontal wells drilled laterally from subsurface tunnels into the lower portion of a tar sand formation is disclosed in U.S. Pat. No. 4,463,988. The described process injects a displacing means such as steam into the boreholes to cause hydrocarbons to flow into the lower portion of the lateral borehole and be produced to the surface.
U.S. Pat. No. 4,577,691 discloses a plurality of parallel horizontal wells arranged in a vertical plane whereby a thermal fluid can be injected into upper wells to drive hydrocarbons down from the area of the upper wells to the horizontal wells immediately below and lying in the same vertical plane. U.S. Pat. No. 4,700,779 discloses a pattern of four or more horizontal wells lying parallel to each other in a horizontal plane within a thin reservoir. The wells in a horizontal plane are used with a combination steam and water injection process to sweep oil from one end to the other end of the pattern.
The use of two or more parallel horizontal injection and production wells is disclosed in U.S. Pat. No. 4,598,770. In this reference, two horizontal wells are drilled parallel to each other at the bottom of the hydrocarbon formation. A thermal fluid is injected through one of the horizontal wells and that fluid and hydrocarbons are produced at the other parallel horizontal well. U.S. Pat. Nos. 4,385,662 and 4,510,997 have a disclosure similar to U.S. Pat. No. 4,598,770 except that a hydrocarbon solvent is injected and allowed to soak in a tar sand formation. Thereafter, a driving fluid such as water is injected to drive the formation fluids and solvent to the horizontal production well in U.S. Pat. No. 4,510,997. The method of U.S. Pat. No. 4,385,662 adds a second injection of solvent followed by a soak period before the drive fluid injection.
U.S. Pat. No. 4,022,279 discloses a system for conditioning an oil or gas formation by drilling horizontal spiralling holes from a vertical well. The patent teaches the injection of unnamed "stimulating fluids" into the spiralling wellbores to provide a way to stimulate bore formation area at a predetermined distance around the vertical well than a series of horizontal wells.
It is known that the use of horizontal injection wells increases the areal sweep of a miscible flood. An increase in areal sweep efficiency for miscible solvent floods has been noted for the use of horizontal injection wells over vertical point source injection wells. Please see Chen, S. M., Olynyk, J., "Sweep Efficiency Improvement Using Horizontal Wells Or Tilted Horizontal Wells In Miscible Floods," CIM Paper No. 85-36-62, Edmonton, Canada (June 2-5, 1985), pages 385-400. Chen and Olynyk noted that the greatest percentage increase in sweep efficiency occurred at the most adverse mobility ratios. A similar increase in areal sweep efficiency was noted for horizontal well injections of carbon dioxide versus point-source injection from vertical wells. See Jones, S. E., "Effects Of Horizontal Wellbore Injection Versus Point-Source Injection On The Recovery Of Oil By CO.sub.2," U.S. Department of Energy Report No. DOE/MC/21207-T23, May 1986.
A related process is described in copending U.S. patent application Ser. No. 140,519, filed 1-4-1988, our Docket No. 78,780. The disclosed process injects a solvent through a horizontal well to form a solvent curtain falling through the reservoir. Hydrocarbons banked below the solvent curtain are produced from the bottom of the reservoir.